The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
Accumulators which are essentially pressurized fluid storage devices are common components of hydraulic fluid systems. They serve two related functions in such systems. On one hand, when a supply pump is operating, they function as a reservoir or storage site for excess pumped fluid resulting simply from pumped fluid volume exceeding system fluid consumption. On the other hand, when a supply pump is not operating or system fluid consumption exceeds pumped volume, the accumulator supplies pressurized fluid until the pump re-starts, supplies pressurized fluid while the pump restarts or until pump output exceeds fluid consumption. Thus, accumulators maintain and create both desired fluid pressure and flow in a hydraulic fluid system, improve the match between the instantaneous volume of fluid supplied by the pump and the instantaneous volume of fluid consumed by the system and thereby improve system operation.
Accumulators are a common component of many automatic transmission configurations in which selective flows of hydraulic fluid are utilized to manipulate spool valves and operate actuators, clutches and brakes to sequentially engage desired speed or gear ratios. The majority of automatic transmission accumulators take two forms: a super-atmospheric gas charged accumulator and a spring biased accumulator. In the first design, one face of a free piston in a cylinder is acted upon by the hydraulic fluid and the adjacent region defines a fluid storage volume; the opposite face of the piston and adjacent volume is charged with, for example, super-atmospheric pressurized nitrogen. The compressed (and compressible) gas provides a fluid spring against which the hydraulic fluid acts. The spring biased accumulator replaces the gas with a mechanical compression spring which biases the piston and maintains the pressure of the hydraulic fluid.
Notwithstanding their popularity, these devices each have shortcomings. For example, given the operating pressures of automatic transmissions, the most practical size gas filled accumulator will, as noted above, include a gas charged to a pressure above atmospheric pressure. Over the life of the accumulator, this pressurized gas will slowly leak out, rendering the accumulator without optimal functionality. This slow change will slowly but inexorably affect the operation of the transmission where there may not be enough fluid storage volume for operations such as re-engaging the clutches for engine start—stop vehicle launches. The alternative to a super-atmospheric pressure charged accumulator is an atmospheric pressure charged accumulator but this choice results in a much larger accumulator which is especially undesirable given the current trend toward highly efficient packaging. A spring accumulator is also generally larger than a gas filled accumulator and thus suffers from the same packaging related problems. Though size may appear to be a minor issue, it is a major issue and has major consequences in automotive component design. Thus, there is a need for an efficiently packaged accumulator for use in hydraulic systems such as those in automatic transmissions.